This invention relates to delignifying pulp in the presence of oxygen, and more particularly to an apparatus and process for the efficient addition, removal, and recycle of oxygen gas in a pulp delignification system.
Oxygen delignification can be carried out on a wide variety of fibrous materials including wood chips and pulp. When carried out on a bleachable grade of pulp, the process is generally referred to as oxygen bleaching. Conventional apparatuses and processes for the oxygen delignification of fibrous material such as cellulosic pulps generally react the materials in a pressurized vertical vessel. One of the problems encountered in oxygen delignification systems is that the partial pressure of oxygen in the vessel is reduced by the presence of air which enters the vessel with the pulp and by other gases which are produced during delignification such as carbon dioxide, carbon monoxide, and hydrocarbon gases. Depending upon the purity of the oxygen gas used, inert gases such as nitrogen and argon may also be introduced along with the oxygen gas into the reaction vessel. The reduced partial pressure of oxygen can have a detrimental effect on delignification resulting not only in a slower reaction rate, but also a reduction in pulp brightness, strength, and other properties. Additionally, the presence of combustible gases such as carbon monoxide and hydrocarbons can be dangerous if their concentration reaches or rises above the lower explosive limit.
One method of increasing the partial pressure of oxygen in the reaction vessel is to increase the operating pressure used for the reaction. However, increased operating pressures require thicker-walled, and therefore more expensive, vessels. Additionally, the danger of gas leakage from the vessel is increased, and the feeding of the pulp into the vessel against this higher pressure becomes more difficult.
Alternatively the partial pressure of oxygen can be increased and the partial pressures of other gases reduced by bleeding gas from the reaction vessel and replacing it with oxygen. However, this procedure increases oxygen usage and removes heat from the vessel. In order to minimize the loss of oxygen resulting from bleeding, it is possible to oxidize catalytically the organic product gases formed during the delignification reaction and recycle at least a portion of the bleed stream back to the reactor vessel while still maintaining the concentration of combustible gases below the lower explosive limit.
Temperature control during oxygen delignification can also be a problem due to the exothermic nature of the reaction. Generally, the pulp must be preheated prior to its entry into the reactor to a temperature sufficiently high to initiate the oxidation reaction. However, once initiated, the heat evolved during the reaction must be controlled to prevent pulp degradation which results from too much heating. This over-heating problem is especially acute for processes designed to generate a large Kappa number decrease (i.e., 30 units or more) in the pulp.
Circulation and cooling of the reactor gas has been used as a method of controlling the temperature within the reactor vessel when operating with high consistency pulp. For example, Hillstrom et al, Svensk Paperstid, Vol. 80, pp. 167-70 (Apr. 10, 1977), teach bleeding gas from the top of a vertical delignification reaction vessel to control the content of carbon monoxide and organic gases therein. The carbon monoxide and organic components of the gas are then catalytically oxidized and the gas cooled and recycled back to the reactor vessel.
Carlsmith, U.S. Pat. No. 3,964,962, teaches withdrawing a portion of gas from a vertical delignification reactor vessel and recycling it back to the upper portion of the reactor. It is taught that the withdrawn gas may be optionally cooled, and the system provides a means to redistribute and control heat within the vessel. Laakso et al, U.S. Pat. No. 4,177,105, teaches a similar gas cooling and recycle system for a vertical delignification reactor vessel. Finally, Luthi et al, in a paper entitled "Gas Concentration and Temperature Distribution in Oxygen Delignification," presented at the 1977 TAPPI Alkaline Pulping Conference, Washington, D.C. Nov. 7-10, 1977, studied both concurrent and countercurrent gas recycle schemes for a vertical oxgen delignification vessel.
However, there are several problems inherent in attempting to control both the partial pressure and temperature of oxygen gas in a conventional vertical delignification reactor. Luthi et al, supra, found that the use of countercurrent gas recirculation to achieve adequate temperature control required large gas flows to avoid undesirable hot spots in the vessel and could result in pulp hang-ups. With respect to concurrent gas recirculation, Luthi et al concluded its use for purposes of temperature control is limited by the compaction of pulp which occurs in the reactor vessel. Additionally, in order for concurrent gas movement to occur at a speed greater than the speed of the pulp, the pulp must be of a high (i.e., 30%) consistency. It is well known, however, that high consistency operation can lead to large temperature increases in the pulp during delignification because of the presence of less dilution water to absorb the heat generated.
Finally, movement of gas through a vertical column of pulp such as is present in conventional high consistency delignification systems may not be uniform. Gas channeling can occur which can lead to hot spots and poor gas distribution in the vessel resulting in pulp degradation and/or an increased danger that combustion will occur. Vertical upflow reactors used for low or medium consistency oxygen delignification, such as those disclosed by Richter, U.S. Pat. No. 4,093,511, Roymoulik, U.S. Pat. No. 3,832,276, and Annergren et al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-14, 1979, pages 99-105, are especially susceptible to channeling of gas and pulp up through the reactor leading to nonuniform gas and termperature distribution. The channeling of pulp in this type of system is illustrated by the residence distribution curve for the 10 ton/day pilot system used by Annergren et al which shows a broad range of residence times for pulp in the reactor as well as an actual means residence time substantially less than the theoretical residence time. This channeling problem can be expected to be much worse for a larger diameter commercial size reaction vessel.
Attempts have been made to modify vertical upflow reactors of the type described above to avoid channeling problems. However, the equipment used to accomplish this is extremely complex as shown by Sherman, U.S. Pat. No. 4,161,421. Moreover, these vertical upflow reaction systems have the additional disadvantage of the inability to circulate gas through the reactor for temperature control since the gas is present as a dispersed phase and travels upwardly at the same speed as the pulp. Jamieson, U.S. Pat. No. 3,754,417, has suggested other reactor designs for oxygen delignification at low pulp consistency. However, those systems also have serious channeling problems and require large inputs of heat because of the low consistency operation.
Accordingly, the need exists in the art for an improved means of supply and recirculation of gas in an oxygen delignification system. The need is especially acute for those systems in which large amounts of delignification are desired since the amount of heat and quantity of combustible and diluent gases generated will be large.